This disclosure is related to the field of evaluation of subsurface formations to determine mineral content, fluid content and fluid productivity. More specifically, the disclosure relates to methods based on measurements of petrophysical parameters for determining positions from within subsurface formations from which samples may be taken from within a wellbore or from a whole drilled core extracted from a wellbore.
Petrophysical and/or geological data may be obtained from subsurface geologic formations during and/or after drilling a wellbore through such formations. These data may be acquired in-situ by wireline, logging while drilling (LWD), measurement while drilling (MWD), cased-hole or other measurements made by moving sensors along the interior of a wellbore. Data may also be acquired using mud logs, laboratory measurements made on cores (whole cores, rotary drilled sidewall cores and/or percussion sidewall cores), drill cuttings, or other representative samples of selected subsurface formations.
When conducting a coring program it is often necessary to select a number of samples for laboratory analysis or qualitative observation (i.e., thin section microscopy). There are methods known in the art to select intervals within the wellbore from which to cut plugs from whole cores (cores drilled using well drilling equipment having an annular drill bit) or to select intervals in the wellbore to cut rotary sidewall cores (RSWCs) or to obtain percussion-type sidewall cores. Such selection methods include visual observation of the above described types of data in unprocessed form and/or after manual or computer implemented interpretation. Such selection techniques are prone to error and subject to individual interpretation, this including substantial subjective basis for selection of sample intervals. It is possible that without proper analysis of the above data important statistical members may be missed entirely in the data set that is to be constructed by the coring program (via subsampling). It is equally possible that the distribution of data collected may be skewed or not statistically representative of the core (actual data set).